Wouldn't it in that case translate more correctly to "let's come at the onsen more!" though? Or is it that since iku linked with a noun by de will always mean "going by means of [noun]", even if the implied meaning of "to go" here is "to climax" and quite distinct from the original meaning, and the noun in this case is a location you can't use as a means of going in even the most abstract of senses? (why the Japanese chose "to go" instead of "to come" for this purpose would be an interesting question too)

Or formulated a bit differently, it really does mean "getting off with the onsen as tool" instead of "getting off at the onsen", i.e. the meaning of the particle does not change with the meaning of the verb?

Just aksing. Only one year of Japanese here, so I'm still not all too familiar with the various nuances of using certain joshi in certain contexts. But I want to learn...